Revert "[gn] Deprecate copy_trees()"

This reverts commit f60d32167f.

Reason for revert: build failures in post submit

Original change's description:
> [gn] Deprecate copy_trees()
>
> Part of https://github.com/flutter/flutter/issues/144430
>
> Previously, this code did backflips to accomplish two goals
> 1. Collect the paths of all files that would be copied from one place to another after applying regex filters so that they could be listed as the "inputs" to a GN action.
> 2. Arrange so that the python script doing the above would only be invoked once during GN to reduce the cost of calling out to python.
>
> However, this is exactly the use-case for the "depfile" parameter to a GN action. Instead of running the script during GN to populate the "inputs" list, when we run the copy_tree.py script, we can instead ask it to generate a depfile to collect all the input files that were actually copied after applying the regex filter. Using that, we don't have to run the python script at all during GN.
>
> TEST=it builds
>
> Change-Id: I41a251ce4659119cdc3997bb2d6fc7ee0831bb6d
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/356481
> Reviewed-by: Siva Annamalai <asiva@google.com>
> Commit-Queue: Zach Anderson <zra@google.com>
> Reviewed-by: Alexander Aprelev <aam@google.com>

Change-Id: Ie4409ed93a3c990fbce375dba1f2d9a3c735ea3f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/356308
Auto-Submit: Zach Anderson <zra@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Zach Anderson <zra@google.com>

utils/dartdev/BUILD.gn

@@ -37,9 +37,14 @@ application_snapshot("generate_dartdev_snapshot") {
   output = "$root_gen_dir/dartdev.dart.snapshot"
 }
 
-copy_tree("copy_prebuilt_devtools") {
-  visibility = [ ":dartdev" ]
-  source = "../../third_party/devtools/web"
-  dest = "$root_out_dir/devtools"
-  exclude = "{}"
+copy_trees("copy_prebuilt_devtools") {
+  sources = [
+    {
+      target = "copy_prebuilt_devtools"
+      visibility = [ ":dartdev" ]
+      source = "../../third_party/devtools/web"
+      dest = "$root_out_dir/devtools"
+      ignore_patterns = "{}"
+    },
+  ]
 }